For illustrating background art in the fields of difference-frequency generation, femtosecond (fs) lasers and enhancement cavities, reference is made to the following prior art documents.    [1] A. Schliesser et al. in “Nature Photonics” vol. 6, 2012, p. 440;    [2] O. Pronin et al. in “Opt. Lett.” vol. 36, 2011, p. 4746;    [3] EP 2 511 751;    [4] EP 2 664 220;    [5] I. Pupeza et al. in CLEO 2014, Postdeadline Paper Digest, paper STh5C.7;    [6] K. L. Vodopyanov et al. in “Optics Express” vol. 22, 2014, p. 4131;    [7] F. Adler et al. in “Opt. Lett.” vol. 34, 2009, p. 1330;    [8] N. Leindecker et al. in “Optics Express” vol. 20, 2012, p. 7046;    [9] F. Keilmann et al. in “Journal of Infrared, Millimeter, and Terahertz Waves” vol. 33, 2012, p. 479;    a[10] A. Gambetta et al. in “Opt. Lett.” vol. 38, 2013, p. 1155;    [11] A. Shiffrin et al. in “Nature” vol. 493, 2013, p. 70;    [12] M. Seidel et al. in “CLEO 2014” Postdeadline Paper Digest, paper STh5C.9;    [13] V. Petrov in “Optical Materials” vol. 34, 2012, p. 536;    [14] E. B. Petersen in “Opt. Lett.” vol. 35, no. 13, 2010, p. 2170;    [15] J. R. Paul et al. in “Opt. Lett.” vol. 38, no. 18, 2013, p. 3654;    [16] R. A. Kaindl et al. in “Appl. Phys. Lett.” 75, 1999, p. 1060;    [17] R. A. Kaindl et al. in “Opt. Lett.” 23, 1998, p. 861;    [18] C. Erny et al. in “Opt. Lett.” 32, 2007, p. 1138; and    [19] CN 102 879 969 A.
Broadband MIR radiation has multiple applications in physics and chemistry, e.g. for measuring purposes or analytics. As an example, broadband MIR radiation is useful for generating few-cycle pulses for time-resolved experiments in physics, e.g. for controlling the properties of transparent dielectrics with light (see [11]). In this case radiation of long wavelength will be capable of transferring electrons between well-separated electrodes (several μm), important for realizing a prototype of a fast switch. Another example of the use of broadband MIR radiation is optical breath analysis aimed at fast non-invasive health diagnostics: all gas molecules have fundamental (i.e. very strong) absorption lines, holding thus a promise to detect them even in small concentrations, down to one part per trillion. The sensitivity of such a method increases with increasing MIR power.
It is a generally known practice to create narrowband or broadened MIR radiation using the non-linear process of optical parametric amplification (OPA) in an optically non-linear crystal. OPA is a coherent process allowing for the energy transfer of a higher-frequency spectral component (pump) to a lower-frequency spectral component (signal) intersecting in the crystal. Energy conservation leads to the generation of a third spectral component (idler) with a frequency corresponding to the difference of the first two. In the particular case, in which energy transfer of both signal and pump radiation to the idler is of interest, this non-linear process is referred to as difference-frequency generation (DFG). Conventionally, the pump light has a higher power compared with the signal light. If the signal light has a power which approximately is equal to that of the pump light, other terms can be introduced and the signal light, pump light and idler light (referring to OPA) correspondingly are called in the following first, second and third spectral components (referring to DFG).
If the optically non-linear crystal for OPA is placed in a laser resonator, this device is referred to as an optical parametric oscillator (OPO). Most of the generally known OPO schemes fall into the following categories: first one, called the singly resonant OPO, has highly reflective mirrors either for the signal or the idler wave. The second one, called the doubly resonant OPO, has two versions: with i) highly reflective mirrors for both signal and idler and ii) highly reflective mirrors for the pump and idler. The existing OPO systems cover the range of interest between e.g. 4 to 14 μm by tuning the generated narrowband (less than 1 μm) spectrum [6].
The output power in such a narrowband mode of operation can reach 1.5 W [7]. The most broadband (but very structured) supercontinuum realized so far in the doubly resonant OPO, reached 37 mW at the −30 dB level; in the range of 2.6 to 6.1 μm [8].
Although the spectral range up to 20 μm could be covered with several OPOs, the OPO approach itself does not allow to make it both broadband and powerful at the same time. This results in particular from the inherent problem that there are no low-loss broadband mirrors in the mid-infrared. Metal mirrors can support broad mid-infrared spectrum, at the expense of losses of the order of 1% per bounce. This means that it is very problematic to build a high-finesse double-resonant OPO cavity and to generate high-power broadband mid-infrared by using the OPO approach.
A direct way of generating broadband MIR radiation by using 1-octave fundamental spectrum from a fs fiber laser and nonlinear fiber in conjunction with a DFG crystal has been proposed, e.g. in [16] or [1]. A phase-stable train of ultrashort laser pulses (fs laser pulses) is described in frequency space as a frequency comb consisting of frequency components (“teeth” of the comb). The fs laser pulses simultaneously contain spectral components in different frequency ranges. According to [1], difference frequencies can be generated among the spectral components (teeth of the single comb). This concept has been experimentally shown e.g. by I. Pupeza et al. [5], where 24 fs laser pulses have been used for driving a DFG process in a DFG crystal resulting in MIR radiation having a power of about 50 mW to 100 mW in a range of about 6 μm to 13 μm. According to [15], the DFG crystal is not arranged in an enhancement cavity. Further experiments resulted in mW-level spectra from 8 to 18 μm [9, 10].
The practical application of the conventional creation of broadband MIR radiation using DFG with fs laser pulses is restricted as a result of the relatively low power of the MIR radiation in the mW range. This disadvantage is caused by the fact that two desirable properties of high power and broad bandwidth usually impose opposite requirements to the nonlinear crystal. To obtain high power, the interaction volume of the different radiation components in the crystal needs to be maximized. Due to chromatic dispersion (and birefringence) this limits the optical bandwidth, over which efficient phasematching can be achieved, resulting in a narrowband idler beam. Customarily, to obtain high power at a given idler frequency, the orientation of the crystal with respect to the pump and signal beams is tuned. To overcome this complication and increase the phasematching bandwidth, the thickness of the crystal can be reduced. This, however, comes at the cost of a significantly reduced generated MIR power.
Driving a DFG process with increased power has been proposed in [14], wherein the DFG crystal has been arranged in an optical cavity. Nanosecond pulses from two separate laser pulse sources having different output wavelengths are coupled into the optical cavity, where the DFG process is obtained with an enhancement factor of 7. Similarly, a continuous wave, single-frequency terahertz (THz) source emitting 1.9 THz has been proposed in [15], wherein two separate laser sources drive the DFG process within the non-linear crystal located in an optical cavity. However, these techniques provide narrowband THz radiation only, i.e. narrowband wavelengths in a range of up to 1 mm. As a further disadvantage, the use of two separate laser pulse sources result in structural complexity of the optical setup.
Further schemes of generating mid-infrared light are described in [17] to [19]. As a limitation, these techniques are also restricted to the generation of tunable narrowband radiation in the mid-infrared range, e.g. in the range of 9 to 18 μm [17] or 3.2 μm to 4.8 μm [18], having relatively low efficiency and power.